Centrifugation

Question 1

What are the advantages of centrifugation

Answer 1

• Continuous processing
• Large capacity in a small volume
• Short residence time (more effective)
• Steam sterilisable
• Automatic operation and cleaning
• Sterile processing
• Contained processing

Question 2

What are the disadvantages of centrifugation

Answer 2

• Large capital investment (expensive)
• Supernatant is not cell free
• Cell rupture may occur
• Large amount of heat produced due to friction

Question 3

What is the force balance on a particle in a centrifuge

Answer 3

F = Fe + Fb + Fd+Fi

Fi is usually ignored as the assumption is that there is a single particle in infinite fluid

Question 4

What is the force Fe, Fb, and Fd on a particle settling under centrifugation and gravity

Answer 4

Fe = wr^2 or mg
Fb = (m/Pp)Pfwr^2 or (m/Pp)Pfg
Fd = CdPfAv^2/2

Question 5

What is centrifugal separation influenced by

Answer 5

• Particle size
• Density difference
• Viscosity of fluid
• centrefuge speed (w^2)

Question 6

What is hindered settling

Answer 6

In narrow channels and high particle concentrations particle - particle and particle - wall interactions may cause deviations from ideal behaviour

Question 7

What are 5 important points about the tubular bowl centrifuge

Answer 7

• Relatively low cost, low capacity
• Fastest industrial machine
• Solids collect at the sides of the rotating cylinder
• Typically used for low batch volumes (low capacity)
• Food dewatering, but limiting solids capacity

Question 8

Key features of a disc stack centrifuge

Answer 8

• Uses extremely high centrifugal forces
• Inserting additional plates aids separation and provides additional settling area
• particle liquid separation is enhanced due to close packed plates at an angle
• used for high cell content sludges

Question 9

Discuss solid discharge from a disc stack

Answer 9

• Solids may be continuously removed from the bowl (using peripheral nozzles)
• Intermittently discharges via hydraulically operated side valve

Question 10

Discuss 4 key points of a scroll centrifuge

Answer 10

• Relatively low G-force (2500-3500)
• used to separate large (>100um) microbial solids
• High broth losses can occur in cell sludge
• Can be used for simultaneous cell separation with solvent extraction (antibiotics)

Question 11

What does the sigma factor physically represent

Answer 11

The cross sectional area of a gravity settler with the same sedimentation characteristics

Question 12

What is the general equation of the sigma equation

Answer 12

(Sigma) = area x Settling relative to gravity